4-Butylresorcinol

This ingredient is also known as rucinol. It's an antioxidant and one of the best-studied tyrosinase inhibitors (skin brightener) in cosmetic chemistry.

This ingredient works in a few complimentary ways:

• It blocks tyrosinase directly by blocking the enzyme from doing its job at creating more melanin
• Inhibits TRP-1. TRP-1 is another enzyme in the melanin pathway. Rucinol disrupts this pathway in case some tyrosinase slips through.
• Helps the cell break down existing tyrosinase faster by tagging tyrosinase molecules for the cell's natural disposal system (ubiquitin–proteasome pathway). This makes the enzyme gets cleared out faster than normal.

In head-to-head biochemical assays, it outperformed kojic acid, arbutin, and even hydroquinone at slowing melanin production.

Clinically, 0.1% has shown to improve melasma visibly after just 4 weeks. Another liposome-encapsulated version performed similarly well and an Indian study on 0.3% showed significant melasma reduction over 8 weeks with no adverse events reported.

This ingredient is generally well-tolerated. A very small amount of people have experienced mild dryness or peeling at a higher concentration (0.3%).

According to a manufacturer, this ingredient is sensitive to air. Good formulations should account for this to keep your rucinol effective.

Be sure to also check out Thiamidol, another "hydroquinone alternative".